This invention relates to the field of magneto-optic recording. More particularly, it relates to improvements in methods and apparatus for optically reading magnetically recorded patterns on a magneto-optic recording element.
The method of optically reading magnetically recorded information by use of the magneto-optic Kerr effect is well known. Such method basically involves steps of irradiating a previously recorded magneto-optic media with a continuous-wave beam of plane polarized radiation (e.g. emanating from a c-w laser), and detecting small clockwise or counter clockwise rotations, typically on the order of 2.degree. or less, in the plane of polarization of the reflected beam. The direction of such rotation, of course, is determined by the state of vertical magnetization (either up or down) of the irradiated magnetic domains representing the recorded information.
Presently preferred magnetic-optic recording elements basically comprise a relatively thick (e.g. one or two millimeters) transparent substrate which supports a relatively thin (e.g. 100 nanometers) layer of magneto-optic recording media. The magneto-optic media may comprise any of a variety of compounds which exhibit a relatively strong Kerr effect, and presently preferred materials include thin films of a transition metal/rare earth alloy. During read-out, the magneto-optic media is irradiated through its transparent substrate.
For a variety of reasons, not the least of which is economy, the transparent substrate of a magneto-optic recording element usually takes the form of a clear plastic (e.g. polycarbonate) disk. Plastic is preferred owing to its capability of being injection molded. While clear plastic disks of relatively high optical quality can be produced by conventional injection-molding techniques, these disks often exhibit a certain amount of stress-induced birefringence which, unfortunately, varies from point-to-point over the disk surface. As the recording element is scanned during read-out, the substrate's varying birefringence has the adverse effect of slightly changing the state of polarization (both angle and ellipticity) of the reflected beam, giving rise to relatively low frequency (e.g. a few KHz.) noise component in the relatively high frequency (e.g. several MHz.) read-out signal. This low frequency noise component tends to amplitude modulate the much higher frequency read-out signal, as well as to bias the average value of such signal to a slowly varying level about zero. While the amplitude modulation of the read-out signal effectively reduces the available output signal, the bias has a more serious effect in that it undermines the common mode rejection technique associated with conventional differential detection schemes.